Method and device for referring a level in a borehole to the bottom of said borehole



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UCL 19, 1943- M. SCHLUMBERGER METHOD AND DEVICE FOR REFERRING A LEVEL-IN A BORE HOLE TO THE BOTTOM OF SAID BORE HOLE Filed Aug. 20, 1938 Patented Oct. 19, 11943 METHOD AND DEVICE Fon. REFERRING A LEVEL IN' A BoREIIoLE To THE oTToM F SAID BoREHoLE Marcel Schlumberger, Paris, France, assignor, by

mesne assignments, to Schlumberger Well Surveying Corporation, Houston, Tex., a corpora tion of Delaware Application August 20, 1938, Serial No. 225,951

Claims.

The present invention relates to methods and apparatus for determining depths, and more particularly to an improved method and apparatus for accurately determining distances in a bore hole from a given reference point.

In the present practice` the depth of a bore hole is usually determined by lowering a plummet on a cable to the bottom of the bore hole, and measuring either the length of cable paid out or the length of cable taken up in raising the plummet to the surface. Likewise, where the bore -hole depth is known, and it is desired to locate apparatus such as a gun perforator in the bore hole at a given distance from the top or bottom thereof, this is often accomplished by lowering the apparatus on a cable to the bottom of the bore hole and taking up a length of cable equal to the distance of the desired location from the bottom of the bore hole.

These methods, however, have been found to be generally unsatisfactory because of the difculty involved in determining the exact moment when the plummet engages or leaves the bottom of the bore hole.

An object of the present invention, accordingly, is to provide an improved method and apparatus for determining depths in a bore hole in which a signal is initiated at the instant the plummet leaves its initial position in the bore hole, thus enabling depth measurements to be made with great accuracy.

A further object of the invention is to provide a method and apparatus of the above character in which the signal is obtained by observing the change in the electrical condition of an electrode als it is moved from its initial position in the bore ole.

A further object of the invention is to provide an improved method and apparatus of the above character in which the signal is initiated by changes in a parameter related to the earth formations surrounding the bore hole in the neighborhood of an exploring electrode as the electrode leaves its initial position in the bore hole.

The signal might also result from a change in the physical conditions and, in particular, in the electrical conditions existing in the apparatus at the exact moment the plummet starts coming up. For instance, it is known that there exists a difference of potential, depending on various factors, between an electrode lowered into a cased or uncased bore hole and the surface of the ground (such factors as electrochemical phenomena, electroltration phenomena in uncased bore holes, etc. as disclosed in prior Patent No.

1,913,293 to Conrad Schlumberger). when this electrode, which may be in one piece with the plummet, is stationary at the bottom of the bore V hole, the said difference of potential has .a-giveny dium'chloride, for instance) so as to modify appreciably the v.salt content of the liquid near the bottom. As set forth in prior Patent No. 1,970,342 to Conrad Schlumberger, the potential of the electrode with respect to a reference point is a function of the salinity of the bore hole liquid. Whenv the electrode rests on the bottom of the hole, it is immersed in this particularly salty'part of the liquid, and acquires a certain potential, which remains constant as long as the electrode remains-motionless. When the electrode begins to move, it leaves this very salt zone and enters the lpart of the liquid not aiected by the dissolved salt. Its potential changes instantly, this constituting a signal as described in the preceding paragraph.

Another process consists in sending a direct current into the ground beforehand by means of the electrode. This direct current electrolyzes the liquid situated in the immediate neighbourhood of the electrode, and consequently changes the composition of this liquid as well as ythe physical condition of the electrode, which becomes polarized. When the current ceases, the electro-chemical equilibrium -formerly destroyed between the electrode and the liquid tends to be re-established, and thus gives rise to a depolarization currentI which can be observed by means of the measuring-apparatuses, and which undergoes a slow and regular evolution as long as the electrode remains motionless, As soon as the electrode rises, it comes into contact with liquid which has not been modified-by electrolysis, and its potential changes abruptly'. This change of potential constitutes a signal lwhich is observed in the measuring-apparatuses by an abrupt change in the depolarization'current.

The signal could also be given by the variations in the electrical resistance of a circuit comprising one or several electrodes carried by the plummet and lowered into the bore hole, said circuit closing by the mud of the bore hole and by the formations which surround the bore hole. Circuits of this general character are illustrated in prior Patents Nos. 1,819,923 and 1,894,328 to Conrad Schlumberger.

However, the signal is created at the moment the plummet leaves the bottom of the bore hole, said signal will be sent direct to the surface by electricity, by means of one or several insulated conductors which will preferably be built into the cable attached to the plummet, and said signal will be recorded at the surface of the ground by any appropriate means.

So as to render the object and scope of the invention more comprehensible, I shall now dei adapted to be used in bore holes not containing a column of conductive liquid.

On Fig. 1, I is a drill hole fitted with a casing 2, the bottom part being stopped up with-a cement plug 3.

The problem to be solved is the following:

The difference in level between the surface of the ground and plug 3 stopping up the bottom of the hole, for instance. is known exactly. The distance between the bottom of the hole and level 4 Where the casing fitting to the walls of the bore hole is to be perforated so as to cause an oil-sand 5 to flow, for example, into the drill hole, is also known.

The perforating apparatus has therefore to be brought to level 4.. It would of course be possible, since the distance between level 4 and the upper end .of Vthe hole is known, to lower this apparatus at the end of a length of cable equal to this distance. But, in practice, when the level to be perforated is situated at a short distance from the bottom of the hole, it is often preferable to bring the perforator to level 4 by taking the bottom of the hole as a reference.

On the figure, 6 is a conventional type perforating apparatus. such as that shown in prior Patent No. 2,048,451, suspended on cable 1, which comprises a certain number of insulated conductors not shown, going from apparatus 6 to the surface of the ground. The outer casing of perforator 6 constitutes an electrode in direct contact with the liquid filling the drill hole. 'An electrode independent of the perforator and connected to one of the insulated conductors mentioned above, may also be used. The electrode (in the case of armoured cable, properly insulated from the armour) is connected by one of the insulated conductors of this cable to potential measuring apparatus 8, which may be a conventional type milllvoltmeter or potentiometer, for example, placed at the surface of the ground, its other pole 9 being earthed at I0.

The other insulated conductors of cable 1 are used to transmit the electric power necessary for the working of perforator 6 (to fire the different powder chambers, for instance, if the perforator is fitted with guns for shooting projectiles through the casing wall) One then proceeds as follows to bring the perforator 6 to level 4.

The apparatus is 4first lowered into the drill hole by means of cable 1, and allowed to rest on the cement plug 3, this operation'being carried out in any known appropriate manner by unwinding the cable from a winch (not shown).

When apparatus 6 is resting on plug 3, the cable is wound up again, while observing .on the measuring-apparatus 8 the position of the needle indicating the diierence of potential betweenV electrode 6 and the earth III.

It is a well known fact in bore hle practice that the potential difference between the lelectrode 6 and the ground point III varies from point to point as the electrode 6 is moved through the bore hole. This is believed to be caused by local action produced by inhomogeneities in the metal of which the casing 4 is constructed, and also to electrochemical effects inthe bore hole.

As long as the perforator 6 rests on the plug 3, this diilerence of potential remains constant. but as soon as apparatus 6 leaves plug 3, a variation in this difference of potential is'observed on' the measuring-apparatus. The cable is then marked, and rewound along a length equal tothe height of level 4 above the plug 3. At this moment, the apparatus is situated exactly opposite the-desired level, and it may be operated.

If the apparatus illustrated in Figure 1 of the drawing is used in a bore hole which is not yet provided with the casing 4, then indications will be obtained of spontaneous potentials between the electrode 6 and the ground point I0. as the electrode 6 is moved through the bore hole. As disclosed in the above mentioned prior Patent No. 1,913,293, these potentials vary from point to point along the bore hole and they are related to the relative porosity of the formations surrounding the bore hole.

Fig. 2`shows the application of the invention to the measurement of the total depth of .a bore hole. The apparatus, which is particularly adapted to the case where the bottom of the hole is uncased, consists in this case, for instance, of a cable comprising three insulated conductors II, I2' and I3, connected to three electrodes I4, I5 and I6, the latter being attached to a body of insulating matter forming a plummet and hanging on the end of the cable. Several representative electrode assemblies of this general character are disclosed in prior Patent No. 1,826,961 to Slichter. Conductors II and I2 are for instance connected at the surfacel of the ground to a source of current I1, conductor I3 being connected to one of the terminals of a potentialdierence measuring apparatus I8, the other terminal of which is earthed at I9.

Electrodes I4 and I5, connected to the source of current I1, have the eiect of creating in the liquid lling the drill hole and in the formations constituting the wall of the drill hole, an electric potential eld, so that'a difference of potential normally exists between electrode I6 and the earth I9, this difference being measured by means of measuring apparatus I8. As disclosed inco-pending' application Serial No. 21,072, filed May 9, 1935, by Conrad Schlumberger, the potential difference between the-electrode I8 and the earth. I9 varies from point to point' along the bore hole as the electrode I6 is moved therethrough, and it is a function of the electrical resistivity of the formations surroundthen the cable is wound up again so as to pull out the assembly.

As long as the plummet. with the three electrodes I4, I and I6 remains at the bottom of the drill hole, the potential difference measured With'apparatus I8 remains constant.

As soon as the plummet leaves the bottom of the hole, this potential difference varies. A mark is made on the cable, which is then wound up so as to bring the plummet to the surface. When thel bottom of this plummet has touched the top edge of the drill hole, the length of cable wound up is measured, and this gives the total length of the drill hole.

In the two above cases. the signal transmitted is created solely by the variation in the electric potential of an electrode.

The description given above refers to the'case where the drill hole is lled with conductive liquid (water or mud). Similar processes may be used according to the present invention in the case of a well either empty, or else filled with an insulating liquid such as oil.

The embodiments shown on Figures l and 2, making use either of the electrochemical or electrofiltration potentials, or of potentials created by the passage of a current sent into the ground to this effect, may also be applied in the case of dry or oil-filled drill holes, the bottom of which is uncased. The electrode or electrodes accompanying the plummet need only be in direct contact with the walls of the drill hole, this bringing about the electrical connection between the apparatuses and the formations.' Electrodes of this kind may be constituted by circular vwire brushes such as that shownin Fig. 3, in contact with the ground, or, if desired, an electrode assembly of the type' disclosed in Figures 5 and 6 of the above mentioned Slichtei patent ,may be used with good results.

Numerous modifications may of course be made in the described device without going beyond the scope of the invention.

In the foregoing specification, and in the appended claims, I have used the word plummet for designating the load supported by a cable. It is to be understood that this load or "plummet may be of any kind, and may be made of any substance, and be of any design, whatsoever.` The only necessary characteristic is that it must be of a weight sumcient to allow the cable to reach the bottom of the hole. In particular, the plummet may be constituted by the apparatus used for certain measurements and operations inside drill holes, such as the above-mentioned gunperforator.

What I claim is:

1. In a method of measuring distances in a bore hole from a reference point located a considerable distance from the top thereof, said bore hole having a potential distribution therein in the vicinity of said reference point which varies abruptly therealong, the steps of disposing an electrode on an elastic cable at said reference point in the bore hole, pulling up said cable at the surface of the earth, thereby producing an initiai elongation thereof .without moving the electrode, and beginning the measurements at the instant the potential difference produced be'- tween the electrode and a groundpoint by said potential distribution beginsto change, whereby errors caused by elastic elongation of the cable may be minimized. y

2. In a. method of measuring distances from the bottom of a bore hole ofV considerable depth, the bore hole having a spontaneous potential dis tribution therein which varies from Apoint 'to point therealong, the steps of disposing an'electrode on an elastic cable at the bottom ofthe bore hole, pulling up said cable at the surface of the earth, thereby producing an initial elongation thereof without moving the electrode, and'beg'ini ning the measurements at thev instant A"the spontaneous potential difference between th` electrode and a ground point begins lto change, whereby errors caused by elastic elongation Vof the cable may be minimized;

3. In a method of measuring distances from the bottom of a bore hole of considerable depth, i* the bore hole being surrounded by earth forma- 'Y tions whose properties vary from point to pointf therealong, the steps of disposing apparatus for obtaining indications of a. parameter of the earth formations surrounding the bore hole on an elastic cable at the bottom of the bore hole, pulling up said cable at the surface ofthe earth, thereby producing an initial elongation thereof without moving the apparatus, and beginning the measurements at the instantv the said parameter characteristic of the formations indicated by the apparatus begins to change, whereby errors caused by elastic elongation of the cable 'mayl be minimized.

4. -In a method of measuring distancesfrom the bottom of a bore hole of considerable depth,

the bore hole being surrounded by earth .'forma-A tions whose electrical resistivity varies from point to point therealong, the steps of disposing apparatus for obtaining indica-tions of the electrical resistivity of the formations on an elastic cable at the bottom of the bore hole, pulling up said cable at the surface of the earth, thereby produc.. ing `an initial elongation thereof without moving the apparatus, and beginning the measurements at the instant the electrical resistivity indicated by the apparatus begins to change, whereby e'rrors caused by' elastic elongation of the cable may be minimized.

5. In a method of measuring distances from the bottom of a, bore hole of considerable depth which contains a column of liquid, the steps of disposing an electrode on an elastic cable at the bottom of the bore hole, changing the ion contom of the bore hole to impart to the electrode a potential different-from its potential in the untreated bore hole liquid, pulling up the cable at the surface of the earth, thereby producing an initial elongation thereof without moving the electrode, and beginning the measurements at the instant the potential of-the electrode with respect to a ground point begins to change, whereby errors caused by elastic elongation of the cable may be minimized. 'i i 6. In a method of measuring distances from the bottom of a bore hole of considerable depth which contains acolumn of liquid, the steps of disposing an electrode on an elastic cable at the bottom ofthe bore hole, passing current from -said electrode into' the surrounding liquid to polarize the electrode and impart to it a potential which is different from its potential in the untreated bore hole liquid, pulling up the cable at the surface of the earth, thereby producing ,anfinitial elongation thereof without moving the electrode, and Y beginningV theV measurements atvthe instant'thefpotential of the electrode with i respect to a groundpointbegins to vary, whereby errorscaused by elastic elongation of thel cable may be'minimized.

V'1.,In a method of measuring distances from the `bottom of -a bore hole ofconsiderable depth which does .not contain a column of conductive liquid, the bore hole having a lpotential distribu- 'moving the electrode, and beginning the measurements at the instant the potential of the electrode with respect to aground point begins to changegwhereby errors caused by elastic elongation of the cable may be minimized. Y

8.V The method of locating a conducting body in a bore hole of considerable depth at a given `distance from the bottom thereof, the bore hole having a potential distribution therein which varies from point to ypoint therealong in the vicinity of the bottom, comprising the steps of loweringv the body .onan elastic cable to the bottom of thebore hole, -pulling up said cable at the surface ofthe earth, thereby producing an initial elongation vthereof without moving the body, be-

.ginning thermeasurenents at` the instant the potential lof the body with respect to a ground point beginsA to change, and-from that instant pulling upa length of cable equal to said given distance, whereby errorscaused by elastic elongation of the cable may be minimized.

9. The method of determining the depth of a .bore hole of considerable depth and having a potential distribution therein which varies from point to ypoint therealong, which comprises the steps of lowering an electrode on an elastic cable to the bottom of the bore hole, pulling up the cable at the surface of the earth, thereby producing an initial elongation thereof without moving the electrode, beginning the measurementsat the instant the potential of the electrode with .respect to a ground point begins to change, and

from that instant measuringl the length of cable pulled up in raising the electrode to the top of the bore hole, whereby errors caused by elastic elongation of the cable may be minimized.

10. The method of determining the depth of a bore hole of considerable depth and surrounded by formations whose electrical propertiesr vary from point to point therealong, which comprises the steps of lowering three spaced apartelectrodes on an elastic cable to the bottom of the bore hole, supplying electrical current to two of said electrodes to set up an electric eld in the surrounding formations, pulling up the cable at the surface of the earth, thereby'producing an initial elongation thereof without moving the electrodes, beginning the depth measurements from the instant the potential of the third electrode with respect to a ground point in said electric field begins to change, and from that instant measuring the length of cable pulled up in raising the electrodes to the top of the bore hole, whereby errors caused by elastic elongation of the cable may be minimized.

MARCEL SCHLUNIBERGER. 

